Positive electrode active material of power storage device, power storage device, electrically propelled vehicle, and method for manufacturing power storage device

ABSTRACT

An object is to improve the characteristics of a power storage device such as a charging and discharging rate or a charge and discharge capacity. The grain size of particles of a positive electrode active material is nano-sized so that a surface area per unit mass of the active material is increased. Specifically, the grain size is set to greater than or equal to 10 nm and less than or equal to 100 nm, preferably greater than or equal to 20 nm and less than or equal to 60 nm. Alternatively, the surface area per unit mass is set to 10 m 2 /g or more, preferably 20 m 2 /g or more. Further, the crystallinity of the active material is increased by setting an XRD half width to greater than or equal to 0.12° and less than 0.17°, preferably greater than or equal to 0.13° and less than 0.16°.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The technical field of the present invention rerates to a power storagedevice and a manufacturing method thereof.

2. Description of the Related Art

With an increase in concern for the environmental issues, energy storagedevices such as secondary batteries and electric double layer capacitorsused for power supply for hybrid vehicles have been actively developed.As the energy storage devices, a lithium ion battery and a lithium ioncapacitor having high energy performance have attracted attention. Thelithium ion battery, which is compact but can store large electricityhas been already mounted on a portable information terminal such as amobile phone or a notebook personal computer, and has helpedminiaturization of products.

The secondary battery and the electric double layer capacitor have astructure in which an electrolyte is provided between a positiveelectrode and a negative electrode. It is known that each of thepositive electrode and negative electrode includes a collector and anactive material provided over the collector. For example, in a lithiumion battery, a material capable of injection and extraction of lithiumions is used in each electrode as an active material, and an electrolyteis interposed between the electrodes.

A lithium oxide and the like are known as a positive electrode activematerial of a lithium ion battery (see Patent Documents 1 and 2).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2008-257894-   [Patent Document 2] PCT International Publication No. 2006/049001

SUMMARY OF THE INVENTION

In a lithium oxide disclosed in Patent Document 1, insertion andextraction of a lithium ion can be performed and change in a crystalstructure is hardly induced by insertion and extraction of a lithiumion; therefore, the lithium oxide is expected as a positive electrodeactive material. However, the conductivity of a lithium oxide is low andthe characteristics thereof have not been improved.

In view of the above problems, an object of one embodiment of thepresent invention is to improve the characteristics of a power storagedevice.

According to one embodiment of the disclosed invention, in a positiveelectrode of a power storage device, a diffusion path of a reactivematerial (e.g., lithium ions) is increased by nano-sizing a grain sizeof an active material.

Alternatively, a diffusion path of a reactive material (e.g., lithiumions) is increased by increasing the surface area per unit mass of anactive material as much as possible. Note that the surface area per unitmass of an active material is increased when surface roughness of theactive material is increased by aggregate of particles having smallgrain sizes.

Further, in one embodiment of the disclosed invention, a lithium oxideis used as a positive electrode active material of a power storagedevice.

As the lithium oxide, a material represented by a general formulaLi_((2-x))MSiO₄ and satisfying conditions (I) and (II) below is used.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (II) M is one or more transition metal elements selected from iron    (Fe), nickel (Ni), manganese (Mn), and cobalt (Co).

In a material represented by the general formula Li_((2-x))MSiO₄, thehighest ratio of lithium atoms to the other metal atoms (M atoms) is 2:1(molar ratio). Therefore, per one composition, the largest number oflithium ions (a reaction material) which can be inserted and extractedis two. When such a material is used as a positive electrode activematerial, higher capacitance can be obtained and the characteristics ofa power storage device can be improved.

In one embodiment of the disclosed invention, an active materialsupported by a carbon material (which is also referred to as acarbon-coated material) is used in a positive electrode. By utilizingthe high conductivity of the carbon material, the internal resistance ofa power storage device is reduced.

One embodiment of the present invention is a positive electrode activematerial of a power storage device, which has a grain size of greaterthan or equal to 10 nm and less than or equal to 100 nm, a surface areaof 10 m²/g or more, and an X-ray diffraction half width of greater thanor equal to 0.12° and less than 0.17°.

Another embodiment of the present invention is a positive electrodeactive material of a power storage device, which has a surface area of10 m²/g or more, and an X-ray diffraction half width of greater than orequal to 0.12° and less than 0.17°.

Another embodiment of the present invention is a positive electrodeactive material of a power storage device, which has a grain size ofgreater than or equal to 10 nm and less than or equal to 100 nm, and anX-ray diffraction half width of greater than or equal to 0.12° and lessthan 0.17°.

Another embodiment of the present invention is a positive electrodeactive material of a power storage device, which has a grain size ofgreater than or equal to 10 nm and less than or equal to 100 nm.

Another embodiment of the present invention is a positive electrodeactive material of a power storage device, which has a surface area of10 m²/g or more.

Another embodiment of the present invention is a power storage deviceincluding a positive electrode, a negative electrode, and an electrolyteprovided between the positive electrode and the negative electrode.Here, an active material provided in the positive electrode has a grainsize of greater than or equal to 10 nm and less than or equal to 100 nm,a surface area of 10 m²/g or more, and an X-ray diffraction half widthof greater than or equal to 0.12° and less than 0.17°.

Further, it is preferable that the above active material be representedby Li_((2-x))MSiO₄ and satisfy (I) and (II) below.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (II) M is one or more transition metal elements selected from iron,    nickel, manganese, and cobalt.

Further, it is preferable that the above active material satisfy atleast one of (III), (IV), (V), and (VI) below.

-   (III) a crystal structure thereof belongs to the space group P1211.-   (IV) a crystal structure thereof belongs to the space group Pmn21.-   (V) a crystal structure thereof belongs to the space group P121/n1.-   (VI) a crystal structure thereof belongs to the space group Pbn21.

Further, it is preferable that the above active material be representedby Li_((2-x))Fe_(s)Ni_(u)SiO₄ and satisfy (I) and (VIII) below.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (VIII) s+u=1, 0≦s≦1, and 0≦u≦1 are satisfied.

Further, it is preferable that the above active material be representedby Li_((2-x))Fe_(s)Mn_(t)Ni_(u)SiO₄ and satisfy (I) and (IX) below.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (IX) s+t+u=1, 0≦s≦1, 0≦t≦1, and 0≦u≦1 are satisfied.

A surface of the above active material may be supported by a carbonmaterial.

Another embodiment of the present invention is a manufacturing method ofa power storage device including a step of forming a positive electrodeand a step of forming a negative electrode facing the positive electrodewith an electrolyte provided therebetween. The positive electrode isformed by a method including the following steps: a step of mixing rawmaterials to be an active material, so that a mixture is formed; a stepof performing first grinding on the mixture; a step of performing firstbaking on the mixture; a step of performing second grinding on themixture; a step of adding saccharide to the mixture, so that a surfaceof the mixture is supported by a carbon material; and a step ofperforming second baking on the mixture at a higher temperature than thefirst baking. Here, the active material has a grain size of greater thanor equal to 10 nm and less than or equal to 100 nm, a surface area of 10m²/g or more, and an X-ray diffraction half width of greater than orequal to 0.12° and less than 0.17°.

Another embodiment of the present invention is an electrically propelledvehicle which is provided with the above-described power storage device.

By microparticulating a positive electrode active material, a diffusionpath of a reactive material can be increased. Thus, a diffusion rate ofthe reactive material and a charging and discharging rate of a powerstorage device can be higher. In other words, the characteristics of thepower storage device can be improved.

In a material represented by the general formula Li_((2-x))MSiO₄, thehighest ratio of lithium atoms to the other metal atoms (M atoms) is 2:1(molar ratio). Therefore, per one composition, the largest number oflithium ions (a reaction material) which can be inserted and extractedis two. When such a material is used as a positive electrode activematerial, higher capacitance can be obtained and the characteristics ofa power storage device can be improved.

When a surface of an active material is supported by a carbon materialwith high conductivity, the internal resistance of a storage device canbe reduced. Therefore, higher voltage can be obtained, which results inhigher discharge capacity. In other words, the characteristics of thepower storage device can be improved.

Further, when microparticulation, application of a material representedby the general formula Li_((2-x))MSiO₄, and support by a carbon materialare performed in combination, the characteristics of a power storagedevice can be significantly improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams each illustrating an example of an activematerial;

FIG. 2 is a graph showing an example of the characteristics of a powerstorage device;

FIG. 3 is a graph showing an example of the characteristics of a powerstorage device;

FIG. 4 is a graph showing an example of the characteristics of a powerstorage device;

FIG. 5 is a graph showing an example of an effect of carbon coating;

FIG. 6 is a graph showing an example of the characteristics of a powerstorage device;

FIGS. 7A to 7C are diagrams each illustrating an example of an activematerial;

FIGS. 8A and 8B are graphs each showing an example of thecharacteristics of a power storage device;

FIGS. 9A to 9C are a table and graphs each showing an example of thecharacteristics of a power storage device;

FIG. 10 is a diagram illustrating an example of a crystal structure of aspace group (P1211) to which Li_((2-x))MSiO₄ belongs;

FIG. 11 is a diagram illustrating an example of a crystal structure of aspace group (Pmn21) to which Li_((2-x))MSiO₄ belongs;

FIG. 12 is a diagram illustrating an example of a crystal structure of aspace group (P121/n1) to which Li_((2-x))MSiO₄ belongs;

FIG. 13 is a diagram illustrating an example of a crystal structure of aspace group (Pbn21) to which Li_((2-x))MSiO₄ belongs;

FIGS. 14A to 14C are diagrams illustrating an example of a structure ofa power storage device;

FIGS. 15A to 15D are diagrams illustrating an example of a method forforming a negative electrode;

FIGS. 16A and 16B are diagrams illustrating an example of a method forforming a negative electrode;

FIGS. 17A and 17B are diagrams each illustrating an example of anegative electrode active material layer;

FIGS. 18A and 18B are diagrams illustrating an example of a method forforming a negative electrode;

FIGS. 19A to 19C are diagrams illustrating an example of a method forforming a negative electrode;

FIGS. 20A and 20B are diagrams each illustrating an example of anelectronic device;

FIGS. 21A and 21B are diagrams each illustrating an example of anelectrically propelled vehicle; and

FIGS. 22A and 22B are graphs each showing a measurement result of X-raydiffraction.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described in detail with reference tothe accompanying drawings. However, since embodiments described belowcan be embodied in many different modes, it is easily understood bythose skilled in the art that the mode and the detail can be variouslychanged without departing from the spirit and the scope of the presentinvention. Therefore, the disclosed invention is not interpreted asbeing limited to the description of the embodiments below. In thedrawings for explaining the embodiments, the same parts or parts havinga similar function are denoted by the same reference numerals, anddescription of such parts is not repeated.

[Embodiment 1]

In this embodiment, a positive electrode active material of a powerstorage device is described.

A material to/from which a reactive material (e.g., lithium ions) can beinserted and extracted is used in a power storage device and thuscharging and discharging can be performed.

A primary requisite for an excellent power storage device is a highcharging and discharging rate.

In one embodiment of the present invention, the grain size of particlesof a positive electrode active material is nano-sized in order toincrease a charging and discharging rate. Alternatively, the surfacearea per unit mass of the positive electrode active material isincreased.

In FIGS. 1A and 1B, examples of a unit mass of an active material 101 inwhich particles 103 are included are illustrated. FIG. 1A illustratesthe case where the particle 103 has large grain sizes and FIG. 1Billustrates the case where the particle 103 has small grain sizes.

In FIG. 1B, as compared to FIG. 1A, the grain size of the particle 103is smaller and the number of the particles 103 included in a unit massof the active material 101 is larger. By increasing the number of theparticles 103, a diffusion path of a reactive material (e.g., lithiumions) in the active material is increased. Thus, a diffusion rate of thereactive material and a charging and discharging rate of a power storagedevice can be higher. Specifically, the grain size of the particle isset to greater than or equal to 10 nm and less than or equal to 100 nm,preferably greater than or equal to 20 nm and less than or equal to 60nm.

The active material 101 in FIG. 1B has a rougher surface than the activematerial 101 in FIG. 1A and thus has a larger surface area per unitmass. By increasing the surface area, a diffusion path of the reactivematerial in the active material is increased. Thus, a diffusion rate ofthe reactive material and a charging and discharging rate of a powerstorage device can be higher. Specifically, the surface area per unitmass is set to 10 m²/g or more, preferably 20 m²/g or more. Note thatwhen the grain size of the particle 103 is reduced, the surface area perunit mass of the active material 101 is increased.

Note that the grain size means a length in a longitudinal direction ofthe particle 103, and an average grain size of the particles included ina unit mass of the active material is referred to as the grain size.Further, the surface area here is measured by a BET method. Note alsothat a reduction in the grain size of the particles of the activematerial or an increase in the surface area per unit mass of the activematerial is also referred to as microparticulation.

In FIG. 2, a relation between the surface area per unit mass of anactive material and rate characteristics in the case where a lithiumoxide is used as a positive electrode active material is shown. Thehorizontal axis indicates a surface area per unit mass (m²/g) of theactive material and the vertical axis indicates rate characteristics.

Here, the rate characteristics which are indicated by the vertical axisare explained. Performing discharging once in an hour is expressed as 1C. Performing discharging n times in an hour is expressed as n C; inother words, one discharging is performed in 1/n hour. Thus, in FIG. 2,it is shown that as 10 C/2 C capacitance in the vertical axis becomeshigher, a charging and discharging rate can be higher. Note that 10 C/2C capacitance means discharge capacity in the case where a dischargerate is 10 C with respect to discharge capacity in the case where adischarge rate is 2 C, and the vertical axis indicates the percentage.

As shown in FIG. 2, by increasing the surface area, the ratecharacteristics can be improved. Note that the rate characteristicschange linearly. This is because a diffusion path of lithium ions isincreased when the surface area becomes larger. Specifically, when thesurface area is 10 m²/g or more, the rate characteristics can be 80% ormore. Further, when the surface area is 20 m²/g or more, the ratecharacteristics can be 85% or more. According to FIG. 2, the ratecharacteristics are favorable when the surface area is at least in arange of greater than or equal to 10 m²/g and less than or equal to 40m²/g, typically greater than or equal to 24 m²/g and less than or equalto 27.5 m²/g. Note that the rate characteristics of 80% or more can beseen as favorable rate characteristics.

Alternatively, the rate characteristics can be improved by reducing thegrain size because a larger surface area results from a smaller grainsize. The grain size is preferably greater than or equal to 10 nm andless than or equal to 100 nm (further preferably greater than or equalto 20 nm and less than or equal to 60 nm).

As described above, by microparticulating an active material, adiffusion path of a reactive material can be increased and a powerstorage device with a high charging and discharging rate can beobtained.

Another requisite for an excellent power storage device is highdischarge capacity.

In one embodiment of the present invention, the crystallinity of anactive material is increased (also referred to as crystallinityenhancement) in order to increase discharge capacity.

In FIG. 3, a relation between the crystallinity of an active materialand discharge capacity in the case where a lithium oxide is used as apositive electrode active material is shown. The horizontal axisindicates an X-ray diffraction half width (°) of the active material andthe vertical axis indicates discharge capacity (mAh/g). The smaller theX-ray diffraction half width (a half width of an X-ray diffraction peak)is, the higher the crystallinity is. Note also that the unit “°”indicates a degree of an angle.

As shown in FIG. 3, when the X-ray diffraction half width is small, thedischarge capacity is high. That is, when the crystallinity is high, thedischarge capacity can be high. The discharge capacity changes linearly.When the X-ray diffraction half width is set to less than 0.2°,specifically, the discharge capacity can be extremely high. In FIG. 3,it is shown that the discharge capacity is high when the X-raydiffraction half width is at least in a range of greater than or equalto 0.12° and less than or equal to 0.2°.

As described above, by increasing the crystallinity of an activematerial, a power storage device with high discharge capacity can beobtained.

Another requisite for an excellent power storage device is having both ahigh charging and discharging rate and high discharge capacity.

In one embodiment of the present invention, an active material ismicroparticulated and the crystallinity of the active material is alsoimproved in order to achieve both a high charging and discharging rateand high discharge capacity.

In FIG. 4, a relation between the crystallinity of an active materialand rate characteristics is shown. The horizontal axis indicates anX-ray diffraction half width (°), which shows the crystallinity. Thevertical axis indicates rate characteristics, which are alreadyexplained above.

As shown in FIG. 4, by decreasing the X-ray diffraction half width, therate characteristics can be improved. Note that when the X-raydiffraction half width is 0.15°, the maximum value of the ratecharacteristics is obtained. That means the existence of the maximumvalue of the crystallinity with respect to the rate characteristics.

When the crystallinity is too low, carrier ions are trapped in grainboundaries between the particles, whereby the mobility of carrier ionsbecomes low and thus the rate characteristics are reduced. On the otherhand, when the crystallinity is too high, it takes time for carrier ionsincluded in one particle to come out and thus the rate characteristicsare reduced.

According to FIG. 4, when the X-ray diffraction half width is set togreater than or equal to 0.12° and less than 0.17°, the charging anddischarging rate can be higher (the rate characteristics of 80% ormore). Further, when the X-ray diffraction half width is set to greaterthan or equal to 0.13° and less than 0.16°, the charging and dischargingrate can be higher (the rate characteristics of 85% or more).

Thus, according to FIG. 2, FIG. 3, and FIG. 4, a power storage devicewith a high charging and discharging rate (the rate characteristics of80% or more) and high discharge capacity can be obtained by setting thesurface area of the active material to be 10 m²/g or more and the X-raydiffraction half width to be greater than or equal to 0.12° and lessthan 0.17°. A power storage device with a higher charging anddischarging rate (the rate characteristics of 85% or more) and higherdischarge capacity can be obtained by setting the surface area of theactive material to be 20 m²/g or more and the X-ray diffraction halfwidth to be greater than or equal to 0.13° and less than 0.16°.

As described above, a high charging and discharging rate and highdischarge capacity can be obtained by microparticulating an activematerial and increasing the crystallinity thereof.

Examples of a method for microparticulation or crystallinity enhancementinclude (1) mixing of materials, (2) baking of a material, and (3)carbon coating of a material. Their details will be described below. Inthe case described below, lithium iron phosphate (LiFePO₄) of an olivinestructure is formed as an example of an active material.

In the first method, microparticulation or crystallinity enhancement isperformed by mixing materials (raw materials).

First of all, as raw materials of lithium iron phosphate, lithiumcarbonate (Li₂CO₃), iron oxalate (FeC₂O₄), and ammonium dihydrogenphosphate (NH₄H₂PO₄) are mixed.

Lithium carbonate is a raw material for introducing lithium; ironoxalate is a raw material for introducing iron; and ammonium dihydrogenphosphate is a raw material for introducing phosphoric acid. Note thatthe present invention is not limited to these raw materials and othermaterials can be used as long as lithium, iron, and phosphoric acid canbe introduced. These raw materials are mixed by a ball mill treatment (afirst ball mill treatment).

The ball mill treatment is performed in such a manner that, for example,a solvent is added, and a ball mill with a ball diameter of greater thanor equal to φ1 mm and less than or equal to φ10 mm is rotated at greaterthan or equal to 50 rpm and less than or equal to 500 rpm for greaterthan or equal to 30 minutes and less than or equal to 5 hours.

When the raw material are mixed by a ball mill treatment,microparticulation of the raw materials can be performed, which leads tomicroparticulation of lithium iron phosphate particles after formation.In addition, by a ball mill treatment, the raw materials can beuniformly mixed and the crystallinity of the lithium iron phosphateparticles after formation can be high.

In the second method, microparticulation or crystallinity enhancement isperformed by baking materials.

By this method, a mixture of raw materials is subjected to two-stepbaking, so that the mixture is crystallized. For example, a mixtureafter the first ball mill treatment by the first method is baked.

Specifically, the mixture is subjected to first baking in a nitrogenatmosphere at a temperature of higher than or equal to 250° C. and lowerthan or equal to 450° C. for greater than or equal to 1 hour and lessthan or equal to 20 hours.

After the first baking, the mixture is ground in a mortar or the like.

Then, the ground mixture is subjected to a ball mill treatment (a secondball mill treatment). The second ball mill treatment is performed insuch a manner that a solvent is added, and a ball mill with a balldiameter of greater than or equal to φ1 mm and less than or equal to φ10mm is rotated at greater than or equal to 50 rpm and less than or equalto 500 rpm for greater than or equal to 30 minutes and less than orequal to 5 hours.

After the second ball mill treatment, the mixture is subjected to secondbaking in a nitrogen atmosphere at a temperature of higher than or equalto 300° C. and lower than or equal to 700° C. for greater than or equalto 1 hour and less than or equal to 20 hours. Note that the temperatureof the second baking is preferably higher than that of the first baking.

After the second baking, the mixture is ground in a mortar or the like.

Then, the ground mixture is subjected to a ball mill treatment (a thirdball mill treatment). The third ball mill treatment is performed in sucha manner that a solvent is added, and a ball mill with a ball diameterof greater than or equal to φ1 mm and less than or equal to φ10 mm isrotated at greater than or equal to 50 rpm and less than or equal to 500rpm for greater than or equal to 30 minutes and less than or equal to 5hours.

By performing two-step baking as described above, a crystal nucleus isformed by the first baking, so that the second baking can be performedin a short time. Thus, crystal growth is suppressed and the grain sizecan be prevented from being increased. That is, microparticulation oflithium iron phosphate particles can be performed. In addition, since acrystal nucleus is formed, the crystallinity can be increased by ashort-time baking.

Note that baking may be performed once instead of performing twice.However, in order to suppress the crystal growth, baking for a long timeat a low temperature is necessary. For example, baking in that casetakes a few days. Therefore, it is preferable that the first baking at alow temperature and the second baking at a higher temperature beperformed similarly to the second method.

In FIG. 5, a relation between baking time for the second baking and thesurface area of lithium iron phosphate (which is plotted with ∘ (whitedots)) and a relation between baking time for the second baking and thecrystallinity of lithium iron phosphate (which is plotted with • (blackdots)) in the case where two-step baking is performed are shown. Thehorizontal axis shows baking time (hour); the left vertical axis, asurface area per unit mass (m²/g); and the right vertical axis, an X-raydiffraction half width, which shows crystallinity. Note that the bakingtime is set to 3 hours, 5 hours, and 10 hours and conditions other thanthe baking time are not varied.

As shown in FIG. 5, when two-step baking is performed so that the bakingtime for the second baking is shortened, the surface area can be large(as indicated by a solid line 501). Further, the crystallinity can behigh even when the baking time is short (as indicated by a dashed line503).

Note that the material may be subjected to a pressure treatment beforebeing subjected to the first baking or the second baking. For example,the material can be shaped into pellets, in which case the density ofthe material is increased. When the density is increased, the number oflithium iron phosphate particles per unit mass after formation of theactive material is increased. In other words, microparticulation can beperformed. Note that when a pressure treatment is performed both beforethe first baking and before the second baking, an effect ofmicroparticulation can be enhanced.

In the third method, by supporting a surface of a material by a carbonmaterial (which is also referred to as carbon coating),microparticulation is performed.

By this method, after a mixture of raw materials is ground, a surface ofthe ground material is supported by a carbon material. For example, amaterial which generates carbon is added to a mixture which is groundafter the first baking by the second method.

Specifically, a material which may generate conductive carbon by thermaldecomposition (hereinafter referred to as a conductive carbon precursor)is added to the ground mixture. As a conductive carbon precursor,saccharide such as glucose or sucrose is added, for example. By adding aconductive carbon precursor, a surface of the mixture is supported by acarbon material. That is, the mixture is carbon-coated.

By adding saccharide as a conductive carbon precursor, many hydroxylgroups contained in saccharide strongly interact with the surface of themixture. Accordingly, crystal growth of the lithium iron phosphateparticles is suppressed. When crystal growth of the lithium ironphosphate particles is suppressed, the grain size can be prevented frombeing increased.

Note that saccharide is preferably added at greater than or equal to 1wt. % and less than or equal to 20 wt. %.

In FIG. 6, a relation between carbon coating and the surface area oflithium iron phosphate is shown. In the horizontal axis, the case wherecarbon coating is performed and the case where carbon coating is notperformed are shown; the vertical axis indicates a surface area (m²/g).Note that conditions other than whether or not carbon coating isperformed are not varied.

As shown in FIG. 6, the surface area in the case where carbon coating isperformed (bar B) can be twice as large as that in the case where carboncoating is not performed (bar A).

When the surface of the ground material is supported by the carbonmaterial as described above, crystal growth of the lithium ironphosphate particles can be suppressed and the lithium iron phosphateparticles after formation can be microparticulated.

Note that a surface of the lithium iron phosphate particles afterformation is also supported by the carbon material which is used here.Support by a carbon material with high conductivity can reduce theinternal resistance of a power storage device and make charge capacityand discharge capacity high. Further, by making discharge capacity high,output per unit mass of the active material can be increased. Output canbe 10 W/g or more, preferably 20 W/g or more.

In other words, the characteristics of a power storage device can beimproved by lithium iron phosphate particles supported by a carbonmaterial. Therefore, support of a material by a carbon material not onlyrealizes microparticulation but also improves the characteristics of theformed particles.

By performing any of the first to third methods, lithium iron phosphateparticles can be microparticulated. By performing the first to thirdmethods as a series of steps, effect of microparticulation becomesfurther more distinctive. However, these formation methods are examplesand the present invention is not limited thereto.

Note that although the case of forming lithium iron phosphate (LiFePO₄)of an olivine structure is described in this embodiment,microparticulation of an active material can be performed by any of thefirst to third methods also in the case of forming other activematerials.

For example, a material represented by a general formulaA_(x)M_(y)PO_(z) (x≧0, y>0, and z>0) can be used as the active material.Here, A represents an alkali metal such as lithium, sodium, orpotassium, or an alkaline earth metal such as beryllium, magnesium,calcium, strontium, or barium. M represents, for example, a transitionmetal element such as iron, nickel, manganese, or cobalt. As thematerial represented by the general formula A_(x)M_(y)PO_(z) (x≧0, y>0,and z>0), lithium iron phosphate, sodium iron phosphate, and the likecan be given. As the material represented by A and the materialrepresented by M, one or more may be selected from the above respectiveexamples.

Alternatively, a material represented by a general formulaA_(x)M_(y)O_(z) (x≧0, y>0, and z>0) can be used as the active material.Examples of such a material include lithium manganate and lithiumcobaltate. As the material represented by A and the material representedby M, one or more may be selected from the above respective examples.

Further alternatively, a material represented by a general formulaA_(x)M_(y)SiO_(z) (x≧0, y>0, and z>0) can be used as the activematerial. That is, a material into which silicate is introduced can beused. For example, lithium iron silicate, lithium iron manganesesilicate, and the like can be given. As the material represented by Aand the material represented by M, one or more may be selected from theabove respective examples. Note that when x=0 in the above generalformula (x≧0), the reactive material (e.g., lithium ions) is completelyextracted.

Note that when M is a plurality of transition metal elements, forexample, M can be two kinds of transition metal elements that aremanganese and iron (when M_(y) is Mn_(s)Fe_(t), s+t=1, 0≦s≦1, and0≦t≦1), two kinds of transition metal elements that are iron and nickel(when M_(y) is Fe_(t)Ni_(u), t+u=1, 0≦t≦1, and 0≦u≦1), or three kinds oftransition metal elements that are manganese, iron, and nickel (whenM_(y) is Mn_(s)Fe_(t)Ni_(u), s+t+u=1, 0≦s≦1, 0≦t≦1, and 0≦u≦1). Here,s+t=1, t+u=1, and s+t+u=1 may be replaced with s+t≈1, t+u≈1, ands+t+u≈1, respectively, due to defects or the like.

When M is two kinds of transition metal elements that are manganese andiron (when M_(y) is Mn_(s)Fe_(t), s+t=1, 0≦s≦1, and 0≦t≦1), i.e., wheniron and manganese whose oxidation-reduction potential is higher thanthat of iron are used, an oxidation-reduction reaction is enhanced andthe charging and discharging characteristics and the like can beimproved.

Further alternatively, the active material may be a solid solutioncontaining a plurality of transition metal elements which are describedabove.

This embodiment can be implemented in combination with any of the otherembodiments or the examples as appropriate.

[Embodiment 2]

In this embodiment, a positive electrode active material of a powerstorage device is described.

A material to/from which a reactive material (e.g., lithium ions) can beinserted and extracted is used in a power storage device and thuscharging and discharging can be performed.

A requisite for an excellent power storage device is high charge ordischarge capacity.

In one embodiment of the present invention, an active material supportedby a carbon material (a material which is carbon-coated) is used in apositive electrode in order to increase charge and discharge capacity.Since the conductivity of a carbon material is high, the internalresistance of a power storage device can be reduced.

Examples of an active material supported by a carbon material areillustrated in FIGS. 7A to 7C. In FIG. 7A, a surface of the particle 103of the active material is partly or entirely covered with a carbonmaterial 105. In FIG. 7B, in a particle group 107 which is formed of theaggregating particles 103, a surface of the particle group 107 is partlyor entirely covered with the carbon material 105. In FIG. 7C, in a layer(which is also referred to as a particle layer 109) formed of aplurality of particles 103, a surface of the particle layer 109 ispartly or entirely covered with the carbon material 105.

The charging and discharging characteristics of a power storage devicein which lithium iron phosphate (LiFePO₄) of an olivine structure isused as an active material are shown in FIGS. 8A and 8B. FIG. 8A showsthe charging characteristics and FIG. 8B shows the dischargingcharacteristics. In both graphs, the horizontal axis indicates capacity(mAh/g) and the vertical axis indicates voltage (V).

FIG. 8A shows the charging characteristics in the case where carboncoating is performed (a solid line 201) and those in the case wherecarbon coating is not performed (a dashed line 203).

FIG. 8B shows the discharging characteristics in the case where carboncoating is performed (a solid line 205) and those in the case wherecarbon coating is not performed (a dashed line 207).

As shown in FIGS. 8A and 8B, the charge capacity and the dischargecapacity in the case where carbon coating is performed are 160 mAh/g andthe charge capacity and the discharge capacity in the case where carboncoating is not performed are 120 mAh/g to 140 mAh/g.

Note that the theoretical capacity in the case of using lithium ironphosphate is 170 mAh/g. That is, in a power storage device in whichlithium iron phosphate supported by a carbon material is used as apositive electrode active material, as much as 94% ((160 mAh/g)/(170mAh/g)×100 (%)) of lithium in the total lithium iron phosphate can bediffused as lithium ions.

When lithium iron phosphate is supported by a highly conductive carbonmaterial, the internal resistance of a power storage device can bereduced, so that charge capacity and discharge capacity become high.

Next, a method of supporting an active material by a carbon material isdescribed below. Here, the case where lithium iron phosphate is formedas the active material is described.

First of all, as raw materials of lithium iron phosphate, lithiumcarbonate (Li₂CO₃), iron oxalate (FeC₂O₄), and ammonium dihydrogenphosphate (NH₄H₂PO₄) are mixed.

Lithium carbonate is a raw material for introducing lithium; ironoxalate is a raw material for introducing iron; and ammonium dihydrogenphosphate is a raw material for introducing phosphoric acid. Note thatthe present invention is not limited to these raw materials and othermaterials can be used as long as lithium, iron, and phosphoric acid canbe introduced.

After that, the mixture is ground in a mortar or the like.

Then, a substance which may generate conductive carbon by thermaldecomposition (hereinafter referred to as a conductive carbon precursor)is added to the ground mixture. As a conductive carbon precursor,saccharide such as glucose or sucrose is added, for example. By adding aconductive carbon precursor, a surface of the mixture is supported by acarbon material. That is, the mixture is carbon-coated.

Note that saccharide is preferably added at greater than or equal to 1wt % and less than or equal to 20 wt %. Note that the film thickness ofthe carbon material supporting lithium iron phosphate after manufactureis preferably greater than 0 nm and 100 nm or less. By performingmicroparticulation, a high discharge potential can be realized, so thatoutput per unit mass of the active material can be 10 W/g or more,preferably 20 W/g or more.

Through the above process, lithium iron phosphate supported by a carbonmaterial can be formed. Further, when a power storage device ismanufactured by using this lithium iron phosphate as an active material,the internal resistance of the power storage device is reduced andcharge capacity and discharge capacity can be high. In addition, whenthe lithium iron phosphate particles are mixed with a conductiveauxiliary agent in a later step, the conductivity can be furtherimproved. Carbon black such as acetylene black can be used as theconductive auxiliary agent. Note that in the case where sufficientconductivity can be obtained by carbon coating, a conductive auxiliaryagent is not necessarily used.

Since lithium iron phosphate can be microparticulated by carbon coating,a diffusion path of lithium ions is increased, so that a charging anddischarging rate of the power storage device can be high.

Note that although the case of forming lithium iron phosphate (LiFePO₄)of an olivine structure is described in this embodiment, charge capacityand discharge capacity can be high by carbon coating also in the case offorming a different active material.

For example, a material represented by the general formulaA_(x)M_(y)PO_(z) (x≧0, y>0, and z>0) can be used as the active material.Here, A represents an alkali metal such as lithium, sodium, orpotassium, or an alkaline earth metal such as beryllium, magnesium,calcium, strontium, or barium. M represents, for example, a transitionmetal element such as iron, nickel, manganese, or cobalt. As thematerial represented by the general formula A_(x)M_(y)PO_(z) (x≧0, y>0,and z>0), lithium iron phosphate, sodium iron phosphate, and the likecan be given. As the material represented by A and the materialrepresented by M, one or more may be selected from the above respectiveexamples.

Alternatively, a material represented by the general formulaA_(x)M_(y)O_(z) (x≧0, y>0, and z>0) can be used as the active material.Examples of such a material include lithium manganate and lithiumcobaltate. As the material represented by A and that represented by M,one or more may be selected from the above respective examples.

Further alternatively, a material represented by the general formulaA_(x)M_(y)SiO_(z) (x≧0, y>0, and z>0) can be used as the activematerial. That is, a material into which silicate is introduced can beused. For example, lithium iron silicate, lithium iron manganesesilicate, and the like can be given. As the material represented by Aand that represented by M, one or more may be selected from the aboverespective examples. Note that when x=0 in the above general formula(x≧0), the reactive material (e.g., lithium ions) is completelyextracted.

Note that when M is a plurality of transition metal elements, forexample, M can be two kinds of transition metal elements that aremanganese and iron (when My is MnsFet, s+t=1, 0≦s≦1, and 0≦t≦1), twokinds of transition metal elements that are iron and nickel (when My isFetNiu, t+u=1, 0≦t≦1, and 0≦u≦1), or three kinds of transition metalelements that are manganese, iron, and nickel (when My is MnsFetNiu,s+t+u=1, 0≦s≦1, 0≦t≦1, and 0≦u≦1). Here, s+t=1, t+u=1, and s+t+u=1 maybe replaced with s+t≈1, t+u≈1, and s+t+u≈1, respectively, due to defectsor the like.

When M is two kinds of transition metal elements that are manganese andiron (when M_(y) is Mn_(s)Fe_(t), s+t=1, 0≦s≦1, and 0≦t≦1), i.e., wheniron and manganese whose oxidation-reduction potential is higher thanthat of iron are used, an oxidation-reduction reaction is enhanced andthe charging and discharging characteristics and the like can beimproved.

Further alternatively, the active material may be a solid solutioncontaining a plurality of transition metal elements which are describedabove.

This embodiment can be implemented in combination with any of the otherembodiments or the examples as appropriate.

[Embodiment 3]

In this embodiment, a material which can be favorably used as a positiveelectrode active material that is one embodiment of the presentinvention is described.

A lithium oxide is used as the positive electrode active material thatis one embodiment of the present invention. In this embodiment, anexample in which silicic acid-based lithium is used as a lithium oxideis described. Detailed description thereof is given below.

A positive electrode active material according to this embodiment isrepresented by the general formula Li_((2-x))MSiO₄ and satisfiesconditions (I) and (II) below.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (II) M is one or more transition metal elements selected from iron    (Fe), nickel (Ni), manganese Mn), and cobalt (Co).

In a material (silicic acid-based lithium) represented by the generalformula Li_((2-x))MSiO₄, the highest ratio of lithium atoms to the othermetal atoms (M atoms) is 2:1 (molar ratio). Therefore, if all thelithium atoms contained in the material can be used for reaction, thelargest number of lithium ions (reaction materials) which can beinserted and extracted can be two per one composition. When such amaterial is used as a positive electrode active material, highercapacitance can be obtained and the characteristics of a power storagedevice can be improved.

A material represented by the general formula Li_((2-x))MSiO₄ can have acrystal structure belonging to a space group in which the highest ratioof lithium atoms to the other metal atoms (M atoms) is 2:1 (molarratio).

For example, when M is iron and x=0, Li₂FeSiO₄ is obtained and can havea crystal structure belonging to the space group P1211 or a crystalstructure belonging to the space group Pmn21. When M is manganese andx=0, Li₂MnSiO₄ is obtained and can have a crystal structure belonging tothe space group Pmn21 or a crystal structure belonging to the spacegroup P121/n1. When M is cobalt and x=0, Li₂CoSiO₄ is obtained and canhave a crystal structure belonging to the space group Pbn21.

In FIG. 10, an example of a crystal structure of Li₂FeSiO₄ which belongsto the space group P1211 is illustrated. The smallest unit of a crystalof Li₂FeSiO₄ which belongs to the space group P1211 includes eightlithium atoms 1001, four iron atoms 1003, four silicon atoms 1005, and16 oxygen atoms 1007. As illustrated in FIG. 10, in Li₂FeSiO₄ whichbelongs to the space group P1211, a ratio of the lithium atoms 1001 tothe other metal atoms (the iron atoms 1003) is 2:1. Thus, it can beknown that in theory, higher capacitance can be obtained in the case ofusing Li₂FeSiO₄ which belongs to the space group P1211 than in the caseof using a material such as LiCoO₂ in which a ratio of lithium atoms tothe other metal atoms is 1:1.

In FIG. 11, an example of a crystal structure of Li₂FeSiO₄ which belongsto the space group Pmn21 is illustrated. The smallest unit of a crystalof Li₂FeSiO₄ which belongs to the space group Pmn21 includes fourlithium atoms 1101, two iron atoms 1103, two silicon atoms 1105, andeight oxygen atoms 1107. As illustrated in FIG. 11, in Li₂FeSiO₄ whichbelongs to the space group Pmn21, a ratio of the lithium atoms 1101 tothe other metal atoms (the iron atoms 1103) is 2:1. Thus, it can beknown that in theory, higher capacitance can be obtained in the case ofusing Li₂FeSiO₄ which belongs to the space group Pmn21 than in the caseof using a material such as LiCoO₂ in which a ratio of lithium atoms tothe other metal atoms is 1:1.

Note that Li₂MnSiO₄ which belongs to the space group Pmn21 can also havethe crystal structure illustrated in FIG. 11. In this case, the ironatoms 1103 in FIG. 11 are replaced with manganese atoms. Further, inLi₂MnSiO₄ which belongs to the space group Pmn21, a ratio of lithiumatoms to manganese atoms is 2:1. Thus, in theory, higher capacitance canbe obtained in the case of using Li₂MnSiO₄ which belongs to the spacegroup Pmn21 than in the case of using a material such as LiCoO₂ in whicha ratio of lithium atoms to the other metal atoms is 1:1.

In FIG. 12, an example of a crystal structure of Li₂MnSiO₄ which belongsto the space group P121/n1 is illustrated. The smallest unit of acrystal of Li₂MnSiO₄ which belongs to the space group P121/n1 includeseight lithium atoms 1201, four manganese atoms 1203, four silicon atoms1205, and 16 oxygen atoms 1207. As illustrated in FIG. 12, in Li₂MnSiO₄which belongs to the space group P121/n1, a ratio of the lithium atoms1201 to the other metal atoms (the manganese atoms 1203) is 2:1. Thus,it can be known that in theory, higher capacitance can be obtained inthe case of using Li₂MnSiO₄ which belongs to the space group P121/n1than in the case of using a material such as LiCoO₂ in which a ratio oflithium atoms to the other metal atoms is 1:1.

In FIG. 13, an example of a crystal structure of Li₂CoSiO₄ which belongsto the space group Pbn21 is illustrated. The smallest unit of a crystalof Li₂CoSiO₄ which belongs to the space group Pbn21 includes eightlithium atoms 1301, four cobalt atoms 1303, four silicon atoms 1305, and16 oxygen atoms 1307. As illustrated in FIG. 13, in Li₂CoSiO₄ whichbelongs to the space group Pbn21, a ratio of the lithium atoms 1301 tothe other metal atoms (the cobalt atoms 1303) is 2:1. Thus, it can beknown that in theory, higher capacitance can be obtained in the case ofusing Li₂CoSiO₄ which belongs to the space group Pbn21 than in the caseof using a material such as LiCoO₂ in which a ratio of lithium atoms tothe other metal atoms is 1:1.

The space group to which a crystal structure of a material representedby the general formula Li_((2-x))MSiO₄ belongs can be controlled byselecting the kind of the metal M atom, a formation method (e.g., abaking temperature), or the like. As examples of the crystal structures,structures (III) to (VI) below can be given.

-   (III) a crystal structure belonging to the space group P1211.-   (IV) a crystal structure belonging to the space group Pmn21.-   (V) a crystal structure belonging to the space group P121/n1.-   (VI) a crystal structure belonging to the space group Pbn21.

In a material represented by the general formula Li_((2-x))MSiO₄, nomatter which space group among the above ones the crystal structurebelongs to, a ratio of lithium atoms to the other metal atoms is 2:1.Therefore, capacitance can be high in theory. When such a material isused as a positive electrode active material, higher capacitance and thelike can be obtained and the characteristics of a power storage devicecan be thus improved.

Note that although examples in which only one kind of element isintroduced as the metal M atom is illustrated in FIG. 10, FIG. 11, FIG.12, and FIG. 13, one embodiment of the present invention is not limitedthereto.

For example, as a positive electrode active material, a materialrepresented by a general formula Li_((2-x))Fe_(s)Mn_(t)SiO₄ andsatisfying conditions (I) and (VII) below can be given.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (VII) s+t=1, 0≧s≧1, and 0≦t≦1 are satisfied.

The above material (which is represented by the general formulaLi_((2-x))Fe_(s)Mn_(t)SiO₄) is obtained by using iron and manganese asthe metal M of a material represented by the general formulaLi_((2-x))MSiO₄. Alternatively, nickel or cobalt may be used as themetal M.

Note that one of the conditions s+t=1 in (VII) may be replaced withs+t≈1 due to defects or the like.

In addition, as a positive electrode active material, a materialrepresented by the general formula Li_((2-x))Fe_(s)Ni_(u)SiO₄ andsatisfying conditions (I) and (VIII) below can be given.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (VIII) s+u=1, 0≦s≦1, and 0≦u≦1 are satisfied.

The above material (which is represented by the general formulaLi_((2-x))Fe_(s)Ni_(u)SiO₄) is obtained by using iron and nickel as themetal M of a material represented by the general formulaLi_((2-x))MSiO₄. Alternatively, manganese or cobalt may be used as themetal M.

Note that one of the conditions s+u=1 in (VIII) may be replaced withs+u≈1 due to defects or the like.

Further, as a positive electrode active material, a material representedby the general formula Li_((2-x))Fe_(s)Mn_(t)Ni_(u)SiO₄ and satisfyingconditions (I) and (IX) below can be given.

-   (I) x is a value changing within a range 0≦x≦2, due to insertion and    extraction of a lithium ion during charging and discharging.-   (IX) s+t+u=1, 0≦s≦1, 0≦t≦1, and ≦u≦1 are satisfied.

The above material (which is represented by the general formulaLi_((2-x))Fe_(s)Mn_(t)Ni_(u)SiO₄) is obtained by using iron, manganese,and nickel as the metal M of a material represented by the generalformula Li_((2-x))MSiO₄. Alternatively, cobalt may be used as the metalM.

Note that one of the conditions s+t+u=1 in (IX) may be replaced withs+t+u≈1 due to defects or the like.

As described above, even in the case where two or more kinds of elementsare used as the metal M atom, the highest ratio of lithium atoms to theother metal atoms is 2:1. When such a material is used as a positiveelectrode active material, higher capacitance can be obtained and thecharacteristics of a power storage device can be thus improved.

This embodiment can be combined with a structure of any of the otherembodiments or the examples as appropriate.

[Embodiment 4]

In this embodiment, an example of a formation method of silicicacid-based lithium is described.

<Formation Method of Li₂FeSiO₄>

An example of a formation method in the case where a metal M atom isiron is described.

Li₂FeSiO₄ can be formed with the use of a raw material for introducinglithium, a raw material for introducing iron, and a raw material forintroducing silicate. For example, lithium carbonate (Li₂CO₃), ironoxalate (FeC₂O₄), and a silicon oxide (SiO₂) can be used as a rawmaterial for introducing lithium, a raw material for introducing iron,and a raw material for introducing silicate, respectively. Further,lithium silicate (Li₂SiO₃) can be used as a raw material for introducinglithium and silicate. Note that the present invention is not limited tothese raw materials and other materials can be used as long as lithium,iron, and silicate can be introduced.

By mixing the materials (the raw materials) and then performing baking,Li₂FeSiO₄ can be formed.

The raw materials can be mixed by a ball mill treatment, for example. Byperforming a ball mill treatment, the raw materials can bemicroparticulated as well as being mixed, so that Li₂FeSiO₄ afterformation can be microparticulated. In addition, by performing a ballmill treatment, the raw materials can be uniformly mixed and thecrystallinity of Li₂FeSiO₄ after formation can be made high.

In the case of mixing the raw materials by a ball mill treatment, theraw materials, a solvent, and balls are put in an apparatus (a ball millpot) and mixing is performed. As a solvent, acetone, ethanol, or thelike can be used. Metallic balls, ceramic balls, or the like can beused. The ball mill treatment can be performed in such a manner that aball mill with a ball diameter of greater than or equal to φ1 mm andless than or equal to φ10 mm is rotated at greater than or equal to 50rpm and less than or equal to 500 rpm for greater than or equal to 30minutes and less than or equal to 5 hours.

For example, lithium carbonate, iron oxalate dihydrate, and a siliconoxide that are raw materials, acetone that is a solvent, and a ballcontaining zirconia (Zr) with a ball diameter of φ3 mm are put in a ballmill pot and rotation is performed at 400 rpm for 2 hours, so that amixture of the raw materials can be formed.

Baking of the mixture of the raw materials can be performed, forexample, at a baking temperature of higher than or equal to 700° C. andlower than or equal to 1100° C. for greater than or equal to 1 hour andless than or equal to 24 hours.

Note that the mixture of the raw materials may be subjected to two-stepbaking of first baking (pre-baking) and second baking (main baking). Thetemperature of the second baking is preferably higher than that of thefirst baking. By performing two-step baking, Li₂FeSiO₄ after formationcan be microparticulated or the crystallinity thereof can be increased.

The first baking can be performed, for example, in a nitrogen atmosphereat a baking temperature of higher than or equal to 250° C. and lowerthan or equal to 450° C. for greater than or equal to 1 hour and lessthan or equal to 20 hours. The second baking can be performed, forexample, in a nitrogen atmosphere at a baking temperature of higher thanor equal to 300° C. and lower than or equal to 700° C. for greater thanor equal to 1 hour and less than or equal to 20 hours.

Note that the mixture of the raw materials may be subjected to apressure treatment before being subjected to the first baking or thesecond baking. For example, the mixture of the raw materials can beshaped into pellets before being baked. In the case where the mixture ofthe raw materials which is shaped into pellets is subjected to two-stepbaking, the mixture shaped into pellets is subjected to the first bakingand the baked product is ground in a mortar or the like and then mixedby a ball mill treatment or the like; the mixture is shaped into pelletsagain and then the second baking can be performed.

For example, a mixture of the raw materials (lithium carbonate, ironoxalate dihydrate, and a silicon oxide) which is formed by mixing by thefirst ball mill treatment is heated to 50° C., so that the solvent(acetone) is evaporated; then, pressure of 150 kgf is applied to themixture with the use of a pellet press for 5 minutes, so that themixture is shaped into pellets. The mixture shaped into pellets issubjected to the first baking (pre-baking) in a nitrogen atmosphere at abaking temperature of 350° C. for 10 hours.

After the baked product (the pre-baked product) is slightly ground, thesecond ball mill treatment is performed in such a manner that the bakedproduct (the pre-baked product), a solvent (acetone), and a ballcontaining zirconia (Zr) with a ball diameter of φ3 mm are put in a ballmill pot and rotation is performed at 400 rpm for 2 hours.

The baked product (the pre-baked product) which is mixed by the secondball mill treatment is heated to 50° C., so that the solvent (acetone)is evaporated; then, pressure of 150 kgf is applied to the baked productwith the use of a pellet press for 5 minutes, so that the baked productis shaped into pellets. The baked product (the pre-baked product) shapedinto pellets is subjected to the second baking (main baking) in anitrogen atmosphere at a baking temperature of 700° C. or 800° C. for 10hours. In this example, by setting the baking temperature of the secondbaking to 700° C., Li₂FeSiO₄ having a crystal structure belonging to thespace group P1211 can be formed. Further, by setting the bakingtemperature of the second baking to 800° C., Li₂FeSiO₄ having a crystalstructure belonging to the space group Pmn21 can be formed.

In Li₂FeSiO₄ which is obtained in the above-described manner, thelargest number of lithium ions which are inserted and extracted can betwo per one composition. When such a material is used as a positiveelectrode active material, higher capacitance can be obtained, which cancontribute to improvement in the characteristics of a power storagedevice.

<Formation Method of Li₂MnSiO₄>

In the case where the metal M atom is manganese, a raw material forintroducing manganese is used instead of a raw material for introducingiron which is used in the above-described formation method of Li₂FeSiO₄.For example, lithium silicate (Li₂SiO₃) and manganese oxalate (MnC₂O₄)can be used as raw materials. By mixing the materials (the rawmaterials) and then performing baking, Li₂MnSiO₄ can be formed.

In Li₂MnSiO₄ which is obtained in the above-described manner, thelargest number of lithium ions which are inserted and extracted can betwo per one composition. When such a material is used as a positiveelectrode active material, higher capacitance can be obtained, which cancontribute to improvement in the characteristics of a power storagedevice.

Although an example in which the metal M atom is iron or manganese isdescribed in this embodiment, a desired material which is represented bythe general formula Li_((2-x))MSiO₄ can be formed by appropriatelyselecting a raw material for introducing the metal M atom.

This embodiment can be combined with a structure of any of the otherembodiments or the examples.

[Embodiment 5]

In this embodiment, an example of a power storage device which uses thepositive electrode active material described in the above embodiment isdescribed.

A part of a structure of a power storage device 2200 is illustrated inFIG. 14A. The power storage device 2200 includes a positive electrode2201 and a negative electrode 2211 which faces the positive electrode2201 with an electrolyte 2207 provided therebetween.

The positive electrode 2201 includes a collector 2203 and a positiveelectrode active material layer 2205 provided over the collector 2203.

An active material (the positive electrode active material layer 2205)of the positive electrode 2201 is formed using the material which isdescribed in the above embodiment. The above embodiment can be referredto for materials and a formation method of the positive electrode activematerial layer 2205. As a material of the collector 2203, a conductivematerial such as platinum, copper, or titanium can be used, for example.

The negative electrode 2211 includes a collector 2213 and a negativeelectrode active material layer 2215 provided over the collector 2213.As a material of the collector 2213, a conductive material such asplatinum, copper, or titanium can be used, for example. As a material ofthe negative electrode active material layer 2215, a carbon materialsuch as graphite, a lithium metal, silicon, or the like can be used.

The electrolyte 2207 has a function of transporting a reactive material(e.g., lithium ions). A material of the electrolyte 2207 can be solid orliquid.

In the case where the material of the electrolyte 2207 is solid, Li₃PO₄,Li_(x)PO_(y)N_(z) (x, y, and z are positive real numbers) which isformed by mixing Li₃PO₄ with nitrogen, Li₂S—SiS₂, Li₂S—P₂S₅, Li₂S—B₂S₃,or the like can be used. Alternatively, any of these which is doped withLiI or the like can be used.

In the case where the material of the electrolyte 2207 is liquid, theelectrolyte includes a solvent and a solute (salt) dissolved in thesolvent. As the solvent, any of cyclic carbonates such as propylenecarbonate and ethylene carbonate, or chain-like carbonates such asdimethyl carbonate and diethyl carbonate can be used. As the solute(salt), for example, a solute (salt) containing one or more kinds oflight metal salts (lithium salt and the like) such as LiPF₆, LiBF₄, orLiTFSA can be used.

Note that a separator 2209 is provided in the case where the electrolyte2207 is liquid. The separator 2209 prevents contact between the positiveelectrode 2201 and the negative electrode 2211 and has a function ofallowing passage of a reactive material (e.g., lithium ions). As amaterial of the separator 2209, for example, paper, nonwoven fabric, aglass fiber, a synthetic fiber such as nylon (polyamide), vinylon (apolyvinyl alcohol based fiber that is also referred to as vinalon),polypropylene, polyester, acrylic, polyolefin, or polyurethane, or thelike can be used. However, a material which does not dissolve in theelectrolyte 2207 should be selected. Further, the separator 2209 may beprovided even when a solid electrolyte is used as the electrolyte 2207.

Next, an example of charging and discharging in the case where a lithiumion secondary battery is used as the power storage device is described.

A power source 2221 is connected between the positive electrode 2201 andthe negative electrode 2211 as illustrated in FIG. 14B, whereby chargingcan be performed. When voltage is applied from the power source 2221,lithium in the positive electrode 2201 is ionized and extracted from thepositive electrode 2201 as a lithium ion 2217 and an electron 2219 isgenerated. The lithium ion 2217 moves to the negative electrode 2211through the electrolyte 2207. The electron 2219 moves to the negativeelectrode 2211 through the power source 2221. Then, the lithium ion 2217receives the electron 2219 in the negative electrode 2211 and isinserted into the negative electrode 2211 as lithium.

In discharging, a load 2223 is connected between the positive electrode2201 and the negative electrode 2211 as illustrated in FIG. 14C, wherebydischarging can be performed. Lithium in the negative electrode 2211 isionized and extracted from the negative electrode 2211 as the lithiumion 2217 and the electron 2219 is generated. The lithium ion 2217 movesto the positive electrode 2201 through the electrolyte 2207. Theelectron 2219 moves to the positive electrode 2201 through the load2223. Then, the lithium ion 2217 receives the electron 2219 in thepositive electrode 2201 and is inserted into the positive electrode 2201as lithium.

As described above, charging and discharging are performed by movementof lithium ions between the positive electrode 2201 and the negativeelectrode 2211. By applying the material described in the aboveembodiment to the positive electrode active material layer 2205 in thepositive electrode 2201 of the power storage device 2200, a highercharging and discharging rate, higher capacitance, and the like can beobtained and the characteristics of the power storage device can be thusimproved.

This embodiment can be implemented in combination with any of the otherembodiments or the examples as appropriate.

[Embodiment 6]

In this embodiment, a power storage device having a structure which isdifferent from that described in the above embodiment will be described.

As a negative electrode active material, a carbon material such asgraphite has been already put to practical use. However, the theoreticalcapacity of a carbon material is limited and it is difficult to obtainhigher capacitance by using such a carbon material which has alreadybeen put to practical use. In this embodiment, a silicon material isused as a negative electrode active material so that the characteristicsof a power storage device are improved.

The material described in the above embodiment is used as a positiveelectrode active material in this embodiment. Thus, the characteristicsof a power storage device can be improved.

Further, a silicon material is used as a negative electrode activematerial in this embodiment. In that case, not only the positiveelectrode active material described in the above embodiment but also thenegative electrode can contribute to improvement in the characteristicsof the power storage device.

Here, a feature of this embodiment is that crystalline silicon is usedas a negative electrode active material so that the characteristics of apower storage device are effectively improved. It is preferable thatcrystalline silicon which is crystallized with the use of a catalyticelement be used as the negative electrode active material. Incrystalline silicon, a diffusion rate of a reactive material (e.g.,lithium ions) is higher than in amorphous silicon; thus, furtherimprovement in the characteristics of the power storage device can berealized in the case of using crystalline silicon.

Crystalline silicon can be obtained by crystallizing amorphous siliconby a heat treatment. At this time, crystallization is preferablyperformed using a catalytic element, in which case a process temperaturefor crystallization can be lowered and process time can be shortened.

By using silicon as a negative electrode active material, highercapacitance can be obtained than in the case of using a carbon material.In addition, by using crystalline silicon in particular, a diffusionrate of carrier ions can be higher than in the case of using amorphoussilicon. Further, when crystalline silicon is formed by a method using acatalytic element, a process temperature for crystallization can belowered and process time can be shortened. In other words, when asilicon layer crystallized with the use of a catalytic element is usedas a negative electrode active material layer, the characteristics of apower storage device can be improved by a formation method which canrealize a reduced manufacturing cost and improved productivity.

A structure and a formation method of a negative electrode is describedwith reference to FIGS. 15A to 15D. As a structure of a power storagedevice, the structure illustrated in FIGS. 14A to 14C can be employedand the negative electrode here corresponds to the negative electrode2211 illustrated in FIGS. 14A to 14C.

An amorphous silicon layer 413 is formed over a negative electrodecollector 411 (see FIG. 15A).

As the negative electrode collector 411, a material with highconductivity, such as titanium, nickel, copper, indium, tin, or silveris used. For example, titanium is used as the negative electrodecollector 411 in this embodiment.

The amorphous silicon layer 413 is formed to a thickness of greater thanor equal to 100 nm and less than or equal to 5 μm, preferably greaterthan or equal to 1 μm and less than or equal to 3 μm by a plasma CVDmethod, a low pressure CVD method, a sputtering method, a vacuumevaporation method, or the like. In the case where the amorphous siliconlayer 413 has a thickness of less than 100 nm, a negative electrodeactive material layer 417, which is obtained after crystallization,becomes so thin that charging and discharging cannot be performed insome cases. In the case where the amorphous silicon layer 413 has athickness of greater than 5 μm, the amorphous silicon layer 413 cannotbe sufficiently crystallized in some cases; in other cases, peeling ofthe negative electrode active material layer 417, which is obtainedafter crystallization, may occur due to stress change at the time ofcharging and discharging. That is why the amorphous silicon layer 413 ispreferably formed to the thickness within the above range.

For example, in this embodiment, the amorphous silicon layer 413 havinga thickness of 3 μm is formed by a plasma CVD method.

A catalytic element 415 for enhancing crystallization is added to theamorphous silicon layer 413 (see FIG. 15B).

As the catalytic element 415, an element which enhances crystallizationof the amorphous silicon can be used. Specifically, a metal element canbe used as the catalytic element 415; for example, one or more elementsselected from nickel (Ni), copper (Cu), indium (In), tin (Sn), andsilver (Ag) can be used. The catalytic element reacts with silicon ofthe amorphous silicon by a later heat treatment so as to form asilicide. The silicide serves as a crystal nucleus so as to contributeto later crystal growth.

For addition of the catalytic element 415, the catalytic element 415 maybe applied on a surface of the amorphous silicon layer 413;alternatively, the catalytic element may be directly attached to thesurface of the amorphous silicon layer 413 by a sputtering method, avacuum evaporation method, or the like.

By adding the catalytic element 415 to the amorphous silicon layer 413,a crystallization temperature of the amorphous silicon layer 413 can belowered by approximately 50° C. to 100° C. Besides, time required forcrystallization of the amorphous silicon layer 413 can be shortened toabout one fifth to one tenth.

Note that nickel can be very favorably used as the catalytic element 415for its effect and reproducibility. A nickel silicide is formed whenamorphous silicon is crystallized with the use of nickel and is to serveas a crystal nucleus when the amorphous silicon is crystallized. In thisembodiment, a specific method for adding nickel as the catalytic element415 to the amorphous silicon layer 413 is described with reference toFIGS. 16A and 16B.

As illustrated in FIG. 16A, a solution 416 containing the catalyticelement 415 is added to the surface of the amorphous silicon layer 413.For example, in the case of using nickel as the catalytic element 415,the solution 416 can be a nickel acetate solution, a nickel chloridesolution, a nickel nitrate solution, or a nickel sulfate solution. Inthis embodiment, a nickel acetate solution is used as the solution 416.

As illustrated in FIG. 16B, spin drying is performed using a spinner421. By performing spin drying, the solution 416 containing thecatalytic element 415 can be uniformly kept on the surface of theamorphous silicon layer 413.

Note that a thin oxide layer is preferably formed on the surface of theamorphous silicon layer 413 before the solution 416 is added. This isbecause when the solution 416 contains water, the solution 416 isrepelled by the hydrophobic surface of the amorphous silicon layer 413and the catalytic element 415 cannot be added to the whole surface ofthe amorphous silicon layer 413 in some cases. By forming a thin oxidelayer on the surface of the amorphous silicon layer 413, the surface canhave higher wettability with respect to the solution 416 (the surfacecan be hydrophilic). The oxide layer can be formed, for example, byirradiating the surface of the amorphous silicon layer 413 with UVlight, or processing the surface of the amorphous silicon layer 413 withthe use of an ammonia hydrogen peroxide mixture, ozone water, or thelike. By such a method, an oxide layer is formed extremely thin, wherebythe catalytic element 415 can reach the amorphous silicon layer 413through the oxide layer.

Alternatively, an organic solution such as an octylic acid solution or atoluene solution can also be used as the solution 416. Organic solutionsare preferable because the solutions contain carbon which belongs to thesame element group as silicon and achieve excellent wetting with thesurface of the amorphous silicon layer 413.

The amorphous silicon layer 413 is subjected to a heat treatment (seeFIG. 15C). By this heat treatment, the amorphous silicon layer 413 iscrystallized and a crystalline silicon layer is obtained. Thecrystalline silicon layer which is thus obtained can be used as thenegative electrode active material layer 417. In this manner, a negativeelectrode 419 in which the negative electrode active material layer 417is provided over the negative electrode collector 411 can be obtained(see FIG. 15D).

The heat treatment for crystallization of the amorphous silicon layer413 can be performed in a heating furnace, for example. Alternatively,the heat treatment can be light irradiation such as laser beamirradiation.

When the heat treatment is performed in a heating furnace, a temperaturecan be set to higher than or equal to 450° C. and lower than or equal to750° C., preferably higher than or equal to 550° C. and lower than orequal to 620° C. Further, heating time can be set to greater than orequal to 1 hour and less than or equal to 24 hours, preferably greaterthan or equal to 4 hours and less than or equal to 10 hours. Forexample, the heat treatment is performed at 550° C. for 4 hours.

In the case where laser beam irradiation is employed as the heattreatment as illustrated in FIG. 15C, energy density can be set togreater than or equal to 100 mJ/cm² and less than or equal to 400mJ/cm², preferably greater than or equal to 200 mJ/cm² and less than orequal to 400 mJ/cm²; typically, the energy density can be set to 250mJ/cm². For example, a laser beam (wavelength: 248 nm, pulse width: 20nsec) of a KrF excimer laser is used for performing the heat treatment.

When the heat treatment is performed, the catalytic element 415 moves inthe amorphous silicon layer 413 and serves as a crystal nucleus, therebyenhancing crystallization. For example, in the case of using nickel asthe catalytic element 415, nickel reacts with silicon of the amorphoussilicon by the heat treatment so as to form a nickel silicide. Thenickel silicide serves as a crystal nucleus so as to contribute to latercrystal growth. Thus, crystallization is enhanced, so that a processtemperature for crystallization can be lowered, process time can beshortened, and a reduced manufacturing cost and improved productivitycan be brought about.

Note that the catalytic element 415 remaining on the crystalline siliconlayer serving as the negative electrode active material layer 417 is notnecessarily removed. This is because the catalytic element 415 is ametal element and has conductivity.

Here, when the amorphous silicon layer 413 is crystallized with the useof the catalytic element 415, the catalytic element is segregated on theoutmost surface of the crystalline silicon layer (the negative electrodeactive material layer 417) after crystallization in some cases, whichdepends on a method and conditions of the heat treatment. In this case,in the crystalline silicon layer, the concentration of the catalyticelement 415 is higher in a region closer to the surface of thecrystalline silicon layer in the depth direction (film thicknessdirection). Further, the catalytic element 415 is oxidized by the heattreatment so as to be a conductive oxide. For example, in the case ofusing nickel as the catalytic element 415, a nickel oxide is segregatedon the outmost surface of the crystalline silicon layer.

In FIGS. 17A and 17B, segregation of a conductive oxide 418 on theoutmost surface of the negative electrode active material layer 417 (thecrystalline silicon layer) is illustrated. In FIG. 17A, the conductiveoxide 418 is segregated in the form of particles on the outmost surfaceof the negative electrode active material layer 417. In FIG. 17B, theconductive oxide 418 is segregated in the form of a layer on the outmostsurface of the negative electrode active material layer 417.

The conductive oxide 418 has conductivity. Therefore, segregation of theconductive oxide 418 on the outmost surface of the negative electrodeactive material layer 417 is not particularly problematic. Thus, the useof the catalytic element can bring about advantageous effects on theprocess and make the application of the crystalline silicon layer to thenegative electrode active material layer 417 effective.

Note that in the case of using copper, indium, tin, or silver as thecatalytic element 415, a copper oxide, an indium oxide, a tin oxide, ora silver oxide is segregated as a result of oxidation. Similarly to anickel oxide, these oxides are also conductive oxides and can bringabout the above-described effect.

Through the above steps, the negative electrode 419 can be formed. In anegative electrode according to this embodiment, a crystalline siliconlayer is applied to a negative electrode active material. Thus, highercapacitance can be obtained and the characteristics of a power storagedevice can be improved. Besides, since a crystalline silicon layer whichis formed through crystallization with the use of a catalytic element isapplied, a reduced manufacturing cost and improved productivity can bebrought about. Further, in a positive electrode, the material describedin the above embodiment is used as a positive electrode active material;therefore, both the negative electrode and the positive electrode cancontribute to improvement in the characteristics of the power storagedevice.

This embodiment can be combined with a structure of any of the otherembodiments or the examples as appropriate.

[Embodiment 7]

In this embodiment, a power storage device having a structure which isdifferent from that described in the above embodiment will be described.

In Embodiment 6, an example is described in which a crystalline siliconlayer serving as the negative electrode active material layer 417 isformed in the following manner: the amorphous silicon layer 413 isformed over the negative electrode collector 411; the catalytic element415 is added to the amorphous silicon layer 413; and the amorphoussilicon layer 413 is then crystallized by a heat treatment. In thisembodiment, an example is described in which a crystalline silicon layerserving as a negative electrode active material layer is formed byutilizing the negative electrode collector itself as a catalyticelement.

An amorphous silicon layer 453 is formed over a negative electrodecollector 451 (see FIG. 18A).

The negative electrode collector 451 is formed using a material whichcontains a catalytic element enhancing crystallization of the amorphoussilicon and has conductivity. Any element can be used as the catalyticelement here as long as it is the same as the catalytic element 415; forexample, nickel (Ni), copper (Cu), indium (In), tin (Sn), silver (Ag),and the like can be given. The negative electrode collector 451 containsone or more of the above catalytic elements. Note that the negativeelectrode collector 451 may contain either a simple substance of acatalytic element or an alloy of a catalytic element and anothermaterial. Note that in the case where the negative electrode collector451 contains an alloy, a material which does not form an alloy withlithium is preferably selected. This is because there is a possibilityof a decrease in stability of the negative electrode collector 451itself in the case where a material which forms an alloy with lithium isused.

The amorphous silicon layer 453 is formed in a manner similar to that ofthe above-described amorphous silicon layer 413.

Then, the amorphous silicon layer 453 is subjected to a heat treatment.By this heat treatment, the amorphous silicon layer 453 is crystallizedand a crystalline silicon layer serving as a negative electrode activematerial layer 457 is obtained. Thus, a negative electrode 459 in whichthe negative electrode collector 451 and the negative electrode activematerial layer 457 are stacked is formed (see FIG. 18B).

When the heat treatment is performed on the amorphous silicon layer 453,thermal diffusion of the catalytic element contained in the negativeelectrode collector 451 is caused and the catalytic element moves fromthe negative electrode collector 451 into the amorphous silicon layer453. Thus, crystal growth of the amorphous silicon layer 453 proceedsfrom an interface between the amorphous silicon layer 453 and thenegative electrode collector 451 to the other surface of the amorphoussilicon layer 453. The crystal growth proceeds from the bottom (theinterface between the amorphous silicon layer 453 and the negativeelectrode collector 451) to the top (the other surface) in the depthdirection (film thickness direction) of the amorphous silicon layer 453.Therefore, in the obtained crystalline silicon layer, the concentrationof the catalytic element is lower in a region closer to the top in thedepth direction. In other words, the concentration of the catalyticelement is lower in a region closer to a top surface of the crystallinesilicon layer.

The conditions of the heat treatment which is performed on the amorphoussilicon layer 413 and is described with reference to FIG. 15C may beapplied to the heat treatment her. In this embodiment, since thenegative electrode collector 451 functions as a catalytic element, aprocess temperature for crystallization can be lowered and process timecan be shortened. Further, a reduced manufacturing cost and improvedproductivity can be brought about.

Besides, in this embodiment, a step of adding a catalytic element is notnecessary because the negative electrode collector 451 itself functionsas a catalytic element, which also lead to a reduced manufacturing costand improved productivity.

Note that the catalytic element remaining on the crystalline siliconlayer serving as the negative electrode active material layer 457 is notnecessarily removed. This is because the catalytic element is a metalelement and has conductivity.

Through the above steps, a negative electrode can be formed. Acrystalline silicon layer is applied to a negative electrode activematerial in this embodiment. Thus, higher capacitance can be obtainedand the characteristics of a power storage device can be improved.Further, since an amorphous silicon layer is crystallized with anegative electrode collector itself functioning as a catalytic element,a further reduction in a manufacturing cost and further improvement inproductivity can be brought about. In addition, in a positive electrode,the material described in the above embodiment is used as a positiveelectrode active material; therefore, both the negative electrode andthe positive electrode can contribute to improvement in thecharacteristics of the power storage device.

This embodiment can be combined with a structure of any of the otherembodiments or the examples as appropriate.

[Embodiment 8]

In this embodiment, a power storage device having a structure which isdifferent from that described in the above embodiment will be described.

In this embodiment, a negative electrode includes a collector, an activematerial layer, and a mixed layer of a collector material and an activematerial. The collector is formed using a metal material and a siliconmaterial is used as a negative electrode active material. The mixedlayer is a mixed layer of the metal material and the silicon material.

The silicon material is used as the negative electrode active material.Since the theoretical capacity of a silicon material can be higher thanthat of a carbon material, higher capacitance can be obtained and thecharacteristics of a power storage device can be improved.

Further, since the mixed layer of the collector material and the activematerial is provided between the collector and the active materiallayer, adhesion between the collector and the active material layer canbe increased and electrons can be more easily donated and accepted. Thisalso contributes to improvement in the characteristics of a powerstorage device.

Note that the material described in the above embodiment is used as apositive electrode active material.

A structure and a formation method of a negative electrode is describedwith reference to FIGS. 19A to 19C. As a structure of a power storagedevice, the structure illustrated in FIGS. 14A to 14C can be employedand the negative electrode here corresponds to the negative electrode2211 illustrated in FIGS. 14A to 14C.

An amorphous silicon layer 473 is formed over a negative electrodecollector 471 (see FIG. 19A).

As the negative electrode collector 471, a conductive material such astitanium, nickel, copper, indium, tin, or silver is used.

The amorphous silicon layer 473 may be formed in a manner similar tothat of the above-described amorphous silicon layer 413.

Then, a heat treatment is performed, so that a mixed layer 475 of thecollector material and the active material is formed (see FIG. 19B).

For example, in the case where titanium is used as the negativeelectrode collector 471, a mixed layer of titanium and silicon is formedas the mixed layer 475. The mixed layer of titanium and silicon may be atitanium silicide layer.

A crystalline silicon layer serving as a negative electrode activematerial layer 477 may be formed by crystallizing the amorphous siliconlayer 473 by this heat treatment. A negative electrode 479 in which thenegative electrode collector 471, the mixed layer 475, and the negativeelectrode active material layer 477 are stacked is formed (see FIG.19C).

Note that the heat treatment in FIG. 19B is performed for formation ofthe mixed layer 475. When the amorphous silicon layer 473 is not sohighly crystallized and desired crystallinity is not obtained by thisheat treatment, another heat treatment (which can be laser beamirradiation) for further crystallization may be performed. Further,amorphous silicon, microcrystalline silicon, or the like can be used asthe negative electrode active material layer 477.

As described above, amorphous silicon, microcrystalline silicon, orcrystalline silicon can be used as the negative electrode activematerial layer 477. A diffusion rate of lithium ions in crystallinesilicon is higher than that in amorphous silicon, which can contributeto improvement in the characteristics of a power storage device;therefore, crystalline silicon is preferable.

Through the above steps, a negative electrode can be formed. In thisembodiment, a mixture layer of a collector material and an activematerial is provided between a collector and an active material layerwhich are included in the negative electrode. Thus, the characteristicsof an interface between the collector and the active material layer(adhesion, easiness of donation and reception of electrons, and thelike) are improved, whereby the characteristics of a power storagedevice can be improved. Further, in a positive electrode, the materialdescribed in the above embodiment is used as a positive electrode activematerial; therefore, both the negative electrode and the positiveelectrode can contribute to improvement in the characteristics of thepower storage device.

This embodiment can be combined with a structure of any of the otherembodiments or the examples as appropriate.

[Embodiment 9]

In this embodiment, application of a power storage device according toone embodiment of the present invention is described.

The power storage device can be provided in a variety of electronicdevices. For example, the power storage device can be provided incameras such as digital cameras or video cameras, mobile phones,portable information terminals, e-book terminals, portable gamemachines, digital photo frames, audio reproducing devices, and the like.Moreover, the power storage device can be provided in electricallypropelled vehicles such as electric vehicles, hybrid vehicles, electricrailway cars, working vehicles, carts, wheel chairs, and bicycles.

The characteristics of a power storage device according to oneembodiment of the present invention are improved; for example, highercapacitance and a higher charging and discharging rate are obtained. Byimproving the characteristics of the power storage device, the powerstorage device can be also compact and lightweight. When being providedwith such a power storage device, electronic devices or electricallypropelled vehicles can have a shorter charging time, a longer operatingtime, and reduced size and weight, and thus their convenience and designcan be improved.

FIG. 20A illustrates an example of a mobile phone. In a mobile phone3010, a display portion 3012 is incorporated in a housing 3011. Thehousing 3011 is provided with an operation button 3013, an operationbutton 3017, an external connection port 3014, a speaker 3015, amicrophone 3016, and the like. When a power storage device according toone embodiment of the present invention is provided in such a mobilephone, the mobile phone can have improved convenience and design.

FIG. 20B illustrates an example of an e-book terminal. An e-bookterminal 3030 includes two housings, a first housing 3031 and a secondhousing 3033, which are combined with each other with a hinge 3032. Thefirst and second housings 3031 and 3033 can be opened and closed withthe hinge 3032 as an axis. A first display portion 3035 and a seconddisplay portion 3037 are incorporated in the first housing 3031 and thesecond housing 3033, respectively. In addition, the second housing 3033is provided with an operation button 3039, a power switch 3043, aspeaker 3041, and the like. When a power storage device according to oneembodiment of the present invention is provided in such an e-bookterminal, the e-book terminal can have improved convenience and design.

FIG. 21A illustrates an example of an electric vehicle. A power storagedevice 3051 is provided in an electric vehicle 3050. The electric powerof the power storage device 3051 is controlled by a control circuit 3053to be output and is supplied to a driving device 3057. The controlcircuit 3053 is controlled by a computer 3055.

The driving device 3057 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The computer 3055outputs a control signal to the control circuit 3053 based on an inputdata such as data of operation (e.g., acceleration, deceleration, orstop) by a driver or data during driving (e.g., data on an upgrade or adowngrade, or data on a load on a driving wheel) of the electric vehicle3050. The control circuit 3053 adjusts the electric energy supplied fromthe power storage device 3051 in accordance with the control signal ofthe computer 3055 to control the output of the driving device 3057. Inthe case where the AC motor is mounted, an inverter which convertsdirect current into alternate current is incorporated.

Charging of the power storage device 3051 can be performed by supplyingpower from the external by a plug-in system. When a power storage deviceaccording to one embodiment of the present invention is provided as thepower storage device 3051, a shorter charging time can be brought aboutand improved convenience can be realized. Besides, the higher chargingand discharging rate of the power storage device can contribute togreater acceleration and excellent characteristics of the electricvehicle. Further, when the power storage device 3051 can be reduced insize and weight as a result of improvement in its characteristics, thevehicle can be reduced in weight and the fuel-efficiency can beimproved.

FIG. 21B illustrates an example of an electric wheelchair. A wheel chair3070 includes a control portion 3073 which is provided with a powerstorage device, a power controller, a control means, and the like. Theelectric power of the power storage device is controlled by the controlportion 3073 to be output and is supplied to a driving portion 3075.Further, the control portion 3073 is connected to a controller 3077. Byoperation of the controller 3077, the driving portion 3075 can be drivenvia the control portion 3073 and movement of the wheel chair 3070 suchas moving forward/backward and a turn and speed can be controlled.

Charging of the power storage device of the wheel chair 3070 can also beperformed by supplying power from the external by a plug-in system. Whena power storage device according to one embodiment of the presentinvention is provided as the power storage device, a shorter chargingtime can be brought about and improved convenience can be realized.Further, when the power storage device can be reduced in size and weightas a result of improvement in its characteristics, the user and thewheelchair helper can use the wheel chair 3070 more easily.

Note that in the case where a power storage device is provided inelectric railway cars as electrically propelled vehicles, charging ofthe power storage device can be performed by supplying power fromoverhead wires or conductive rails.

This embodiment can be combined with a structure of any of the otherembodiments or the examples as appropriate.

EXAMPLE 1

In this example, a specific manufacturing method of a power storagedevice where lithium iron phosphate is used as a positive electrodeactive material is described.

As raw materials of lithium iron phosphate, lithium carbonate (Li₂CO₃),iron oxalate (FeC₂O₄), and ammonium dihydrogen phosphate (NH₄H₂PO₄) weremixed by a first ball mill treatment.

Lithium carbonate was a raw material for introducing lithium; ironoxalate was a raw material for introducing iron; and ammonium dihydrogenphosphate was a raw material for introducing phosphoric acid.

The first ball mill treatment was performed in such a manner thatacetone was added as a solvent and a ball mill with a ball diameter ofφ3 mm was rotated at 400 rpm for 2 hours.

After the first ball mill treatment, the mixture of the raw materialswas shaped into pellets by applying pressure of 1.47×10² N (150 kgf).

Next, the mixture which was shaped into pellets was subjected to firstbaking. The first baking was performed at 350° C. for 10 hours with themixture placed in a nitrogen atmosphere.

After the first baking, the baked mixture was ground in a mortar.

After that, glucose was added to the ground mixture and a surface of themixture was supported by a carbon material. The amount of glucose wasvaried within a range of from 5 wt % to 15 wt %.

The mixture to which glucose was added was subjected to a second ballmill treatment. The second ball mill treatment was performed in such amanner that acetone was added as a solvent, and a ball mill with a balldiameter of φ3 mm was rotated at 400 rpm for 2 hours.

After the second ball mill treatment, the mixture was shaped intopellets again. Then, second baking was performed. In the second baking,the mixture was placed in a nitrogen atmosphere, and a temperature wasvaried within a range of from 400° C. to 600° C. and baking time wasvaried within a range of from 3 hours to 10 hours.

After the second baking, the baked mixture was ground in a mortar.

Then, the ground mixture was subjected to a third ball mill treatment.The third ball mill treatment was performed in such a manner thatacetone was added as a solvent, and a ball mill with a ball diameter ofφ3 mm was rotated at 300 rpm for 3 hours.

Through the above steps, lithium iron phosphate particles were obtained.The particles were mixed with a conductive agent, a binder, and asolvent, and were dispersed by using a homogenizer. The dispersedmaterial was applied on a positive electrode collector and dried,whereby a positive electrode active material layer was obtained. Notethat an aluminum foil was used as the positive electrode collector;acetylene black, the conductive agent; a polyvinylidene fluoride, thebinder; and N-methyl-2-pyrrolidone (NMP), the solvent.

Pressure was applied to the dried material, and the shape of the driedmaterial was fixed, whereby a positive electrode was formed. Pressurewas applied with the use of a roll press so that a film thickness wasabout 50 μm and the amount of the carbon material supporting lithiumiron phosphate was about 3 mg/cm², and the positive electrode wassubjected to punching so as to have a round shape with φ12 mm, whereby apositive electrode of a lithium ion secondary battery was obtained.

A lithium foil was used as a negative electrode and polypropylene (PP)was used as a separator. An electrolyte solution in which a solute waslithium hexafluorophosphate (LiPF₆) and a solvent was ethylene carbonate(EC) and dimethyl carbonate (DC) was used. Note that the separator wasimpregnated with the electrolyte solution.

Through the above steps, a coin-type lithium ion secondary batteryincluding the positive electrode, the negative electrode, the separator,and the electrolyte solution was obtained. Assembly of the positiveelectrode, the negative electrode, the separator, the electrolytesolution, and the like was performed in a glove box with an argonatmosphere.

Results of measurement of various kinds (a surface area, ratecharacteristics, an X-ray diffraction half width, and dischargecapacity) and formation conditions of Samples A to G of the obtainedlithium iron phosphate particles are shown in FIGS. 9A to 9C. Note thatthe rate characteristics are indicated by a percentage of dischargecapacity in the case where a discharge rate is 10 C with respect todischarge capacity in the case where a discharge rate is 2 C at adischarge test (constant current drive/constant voltage drive (CCCVdrive)). The rate characteristics indicate a charging and dischargingrate. Further, the X-ray diffraction half width is a half width of anX-ray diffraction peak and indicates crystallinity. The surface area wasmeasured by a BET method. Note that the discharge capacity was measuredby a discharge test at a discharge rate of 0.2 C with CCCV drive.

In FIG. 9B, a relation between the surface area and the ratecharacteristics (which is plotted with (white dots)) and a relationbetween the crystallinity and the rate characteristics (which is plottedwith • (black dots)) are shown. The lower horizontal axis indicates asurface area (m²/g); the upper horizontal axis, an X-ray diffractionhalf width)(°); and the vertical axis, rate characteristics (%).

From FIG. 9B, it was confirmed that the rate characteristics improve asthe surface area is increased. It is to be noted that the ratecharacteristics changed linearly (a solid line 601). The graph showsthat the rate characteristics are favorable when the surface area is atleast in a range of greater than or equal to 24 m²/g and less than orequal to 27.5 m²/g.

In Sample A, although the surface area was large, the ratecharacteristics were low. It can be considered that this resulted fromthe larger addition amount of glucose than the other samples and thecrystallinity was reduced by adding such a large amount of glucose. Inother words, it was confirmed that high rate characteristics can beobtained in the case where glucose is added at greater than or equal to5 wt % and less than or equal to 10 wt % in the manufacturing method ofExample 1.

The rate characteristics of Sample F and Sample G were much lower thanthose of the other samples. It can be considered that this resulted fromthe lower baking temperature than the other samples and thecrystallinity was reduced by the low-temperature baking.

Further, from FIG. 9B, it was confirmed that the maximum value of thecrystallinity (the X-ray diffraction half width) with respect to therate characteristics exists (a dashed line 603). The graph shows thatthe rate characteristics are favorable when the X-ray diffraction halfwidth is at least in a range of greater than or equal to 0.13° and lessthan 0.17°.

In FIG. 9C, a relation between the crystallinity and the dischargecapacity is shown. It was confirmed that the discharge capacity isincreased as the crystallinity is increased (i.e., as the X-raydiffraction half width is decreased). It is to be noted that thedischarge capacity changed linearly (a solid line 605). The graph showsthat the discharge capacity is high when the X-ray diffraction halfwidth is at least in a range of greater than or equal to 0.13° and lessthan or equal to 0.2°.

Note that, as seen from FIG. 9A, a power storage device with high ratecharacteristics and high discharge capacity could be obtained even withthe baking time shortened.

This example can be implemented in combination with any of the otherembodiments as appropriate.

EXAMPLE 2

In this example, formation of lithium iron silicate as a lithium oxideand X-ray diffraction measurement results thereof are described.

First of all, a formation method of the measured lithium iron silicateis described.

As raw materials of lithium iron silicate, lithium carbonate (Li₂CO₃),iron oxalate dihydrate (FeC₂O₄.2H₂O), and a silicon oxide (SiO₂) weremixed by a first ball mill treatment.

The first ball mill treatment was performed in such a manner thatacetone was added as a solvent, and a ball mill with a ball diameter ofφ3 mm was rotated at 400 rpm for 2 hours.

After the first ball mill treatment, the mixture of the raw materialswas taken out of a pot and heated to 50° C., so that acetone wasevaporated. Then, pressure of 1.47×10² N (150 kgf) was applied to themixture of the raw materials with the use of a pellet press for 5minutes, so that the mixture of the raw materials was shaped intopellets.

Then, the mixture which was shaped into pellets was subjected to firstbaking (pre-baking). The first baking was performed in a nitrogenatmosphere at 350° C. for 10 hours.

After the first baking, the baked mixture was ground in a mortar.

After that, the ground mixture was mixed with glucose at 10 wt % of themixture by a second ball mill treatment.

The second ball mill treatment was performed in such a manner thatacetone was added as a solvent, and a ball mill with a ball diameter ofφ3 mm was rotated at 400 rpm for 2 hours.

After the second ball mill treatment, the mixture of the raw materialswas taken out of a pot and heated to 50° C., so that acetone wasevaporated. Then, pressure of 1.47×10² N (150 kgf) was applied to themixture of the raw materials with the use of a pellet press for 5minutes, so that the mixture of the raw materials was shaped intopellets.

Then, the mixture which was shaped into pellets was subjected to secondbaking (also referred to as main baking). The second baking wasperformed in a nitrogen atmosphere at 700° C. for 10 hours.

X-ray diffraction (XRD) measurement was performed on the lithium ironsilicate which was formed as described above. A measurement result ofX-ray diffraction is shown in FIG. 22A. From the result shown in FIG.22A, it was confirmed that the formed lithium iron silicate had acrystal structure belonging to the space group P1211.

Further, a result of X-ray diffraction measurement performed on lithiumiron silicate which was formed by the above-described formation methodin which the baking temperature of the second baking was set to 800° C.is shown in FIG. 22B. From the result shown in FIG. 22B, it wasconfirmed that the formed lithium iron silicate had a crystal structurebelonging to the space group Pmn21. Note that in the result shown inFIG. 22B, a peak which seems to be a peak of a simple substance of ironis observed. It can be assumed that this peak of a simple substance ofiron resulted from iron which was contained in the raw material.

As described above, it was confirmed that either lithium iron silicatewhose crystal structure belongs to the space group P1211 or lithium ironsilicate whose crystal structure belongs to the space group Pmn21 can beformed. Moreover, it was confirmed that by changing the temperature ofthe second baking (main baking), a crystal structure to be formed can beselected from a crystal structure belonging to the space group P1211 anda crystal structure belonging to the space group Pmn21.

This application is based on Japanese Patent Application serial no.2010-104027 filed with Japan Patent Office on Apr. 28, 2010, JapanesePatent Application Serial No. 2010-103832 filed with Japan Patent Officeon Apr. 28, 2010, and Japanese Patent Application Serial No. 2010-104082filed with Japan Patent Office on Apr. 28, 2010, the entire contents ofwhich are hereby incorporated by reference.

What is claimed is:
 1. A manufacturing method of a power storage devicecomprising the step of: forming a positive electrode comprising thesteps of: performing a first grinding on a mixture of raw materials;performing a first baking on the mixture after the first grinding;performing a first pressure treatment before the first baking;performing a second grinding on the mixture after the first baking;adding a saccharide to the mixture after the second grinding, so that asurface of the mixture is supported by a carbon material; performing asecond baking on the mixture after the adding at a higher temperaturethan the first baking, so that a positive electrode active material isformed; and performing a second pressure treatment before the secondbaking, wherein the positive electrode active material has a surfacearea per unit mass of 24 m²/g or more and 27.5 m²/g or less, and anX-ray diffraction half width of greater than or equal to 0.13° and lessthan 0.17°.
 2. The manufacturing method of a power storage deviceaccording to claim 1, the positive electrode active material isrepresented by a formula Li_((2-x))MSiO₄, x is a value changing within arange 0≦x≦2 due to insertion and extraction of a lithium ion duringcharging and discharging, and M is a transition metal element selectedfrom the group consisting of iron, nickel, manganese, and cobalt.
 3. Themanufacturing method of a power storage device according to claim 2, thepositive electrode active material belongs to a space group selectedfrom the group consisting of space group P1211, space group Pmn21, spacegroup P121/n1, and space group Pbn21.
 4. The manufacturing method of apower storage device according to claim 1, the positive electrode activematerial is represented by a formula Li_((2-x))Fe_(s)Ni_(u)SiO₄, x is avalue changing within a range 0≦x≦2 due to insertion and extraction of alithium ion during charging and discharging, and s+u=1, 0≦s≦1, and0≦u≦1.
 5. The manufacturing method of a power storage device accordingto claim 1, the positive electrode active material is represented by aformula Li_((2-x))Fe_(s)Mn_(t)Ni_(u)SiO₄, x is a value changing within arange 0≦x≦2 due to insertion and extraction of a lithium ion duringcharging and discharging, and s+t+u=1, 0≦s≦1, 0≦t≦1, and 0≦u≦1.
 6. Themanufacturing method of a power storage device according to claim 1, thesaccharide is added at greater than or equal to 1 wt % and less than orequal to 14 wt %.
 7. The manufacturing method of a power storage deviceaccording to claim 1 comprising the step of forming a negative electrodeover the positive electrode with an electrolyte interposed therebetween.8. The manufacturing method of a power storage device according to claim1, the second baking is performed in a nitrogen atmosphere at a bakingtemperature of higher than or equal to 550° C. and lower than or equalto 700° C.
 9. The manufacturing method of a power storage deviceaccording to claim 1, the positive electrode active material is lithiumiron phosphate.
 10. The manufacturing method of a power storage deviceaccording to claim 1, comprising the step of: forming a negativeelectrode comprising the steps of: forming an amorphous silicon layerover a negative electrode collector, the amorphous silicon layer havinga thickness of 100 nm and less than or equal to 5 μm; adding nickel tothe amorphous silicon layer; and performing a heat treatment to theamorphous silicon layer so that the amorphous silicon layer iscrystallized.
 11. The manufacturing method of a power storage deviceaccording to claim 10, comprising the step of forming an oxide layer ona surface of the amorphous silicon layer before the adding step.